US3324672A

US3324672A - Electrically controlled conditioning system

Info

Publication number: US3324672A
Application number: US572381A
Authority: US
Inventors: William L Sones; James O Elliott; George A Neyhouse
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1964-08-31
Filing date: 1966-08-15
Publication date: 1967-06-13
Anticipated expiration: 1984-06-13

Description

June 13, 1967 W. L. SONES ETAL ELECTRICALLY CONTROLLED CONDITIONING SYSTEM Original Filed Aug. 31. 1964 3 Sheets-Sheet l INVENTORS William L. Sones James 0. Elliot! Y George A. Neyhouse ,C Their Attorney June 13, 1967 W. L. SONES ETAL ELECTRIGALLY CONTROLLED CONDITIONING SYSTEM Original Filed Aug. 51, 1964 3 Sheets-Sheet 2 SPEEDSENSING 25 MEANS BLOWER A! POWER v 5| MOTOR SOURCE COMPUTER COME 3g CONTROL um'r MOTOR l 5e COMFORT SPEED CONTROL SENSING PROGRAMMER MEANS INVENTORS William L. Spnes James O.EIII0H B George A. Neyhouse ,7 Their Attorney 324,672 ELECTRICALLY CONTROLLED CONDITIONING SYSTEM Original Filed Aug. 51, 1964 June 3, 1957 w. SONES ETAL 3 Sheets-Sheet I5 Ill rx r e COMPUTER O n. m w 7 5 s wmwm m ma -|||-$E mod e m m C m WJG Y B Their Attorney United States Patent Ofi ice 3,324,672 Patented June 13, 1967 3,324,672 ELECTRICALLY CONTROLLED CON- DITIONING SYSTEM William L. Sones, Dayton, James 0. Elliott, Xenia, and George A. Neyhouse, Kettering, Ohio, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Original application Aug. 31, 1964, Ser. No. 393,202, now Patent No. 3,267,994, dated Aug. 23, 1966. Divided and this application Aug. 15, 1966, Ser. No. 572,381 6 Claims. (Cl. 62-180) This application is a division of copending United States application, Ser. No. 393,202, filed Aug. 31, 1964, and now Patent No. 3,267,994.

This invention relates to environmental conditioning systems and more particularly to control systems for heating and/or cooling systems for maintaining predetermined desired environmental comfort conditions.

In conditioning residences, oflice buildings and the like, refrigerant systems and heating systems often have been integrated into a common conditioning system for modulating and distributing air to obtain desired temperatures and humidities within the environment being conditioned.

In such systems it is desirable both from an operating efficiency and a conditioning standpoint to operate the system more or less continuously. While it is desirable to have such a continuous modulation of the air being treated, in the past, for example, refrigerant systems have relied upon a concept of on and off cycling of a refrigerant compressor in response to sensed temperatures to modulate the treated air. Accordingly, during periods of refrigerant system operation, a full on or off condition exists that at times may introduce too much cold or at other times is inadequate to meet the demands of the regions being treated. Likewise, in distributing conditioned air to the regions being treated, typically fan means are provided that are either at a full on or full off condition. Thus, the treated air is usually circulated at a predetermined compromise rate that either fails to satisfactorily distribute needed volumes of conditioned air to meet high demand requirements or is too great when the conditioned region is not subject to extreme changes in temperature or the like.

Because of the above-mentioned operating inflexibility, it has been found that there is either a substantial lag or a like overshoot of conditions in the environment with respect to desirable comfort control points. Moreover, such operation obviously materially reduces operating efficiency.

An object of the present invention, therefore, is to improve the operation of systems for conditioning environments by the provision therein of a static semiconductor computer control means for continuously varying the energization of electric motor means whereby conditioning components of the system are driven to maintain desirable comfort control points in the conditioned region without any substantial lag or overshoot of the control points.

A further object of the present invention is to provide an integrated heating and cooling system including a source of heat, a refrigerant system, means including fan means for distributing air conditioned by said heat source and said refrigerant system wherein the compressor and fan are driven by motor means controlled at a desired speed norm by a solid state computer controller having semiconductor switch means with a power supply conductivity related to a feedback signal of the speed of the compressor motor and further having comfort control programmer means for infinitely varying environmental conditions on a seasonal basis in part by infinitely varying the feedback signal from the electric motor means within predetermined limits.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

FIGURE 1 is a schematic view of the inventive yeararound environmental conditioning system shown in association with a residential housing unit;

FIGURE 2 is a block diagram of the present invention;

FIGURE 3 is an electrical circuit diagram of a solid state computer for practicing the present invention; and

FIGURE 4 is an electrical circuit showing the computer associated with the comfort control programmer of the present invention.

With reference to the drawings in FIGURE 1, an environmental conditioning system is shown in association with a residential household unit 10 representatively illustrated as including a first conditioned region or room 12 and a second conditioned region or room 14 that are separated from one another by interior walls or the like as typically found in such units.

The rooms 12 and 14 are connected by means of supply ducts 16, 18, respectively, to an air supply plenum 20. The air supply plenum 20 will typically be in communication with a return duct 22 (partially shown) for directing air from the regions 12, 14 back into the plenum. Air circulation is maintained by a suitable fan 24 driven by a blower motor 25 for drawing air from the return duct 22 and forcing it under pressure through the supply ducts 16, 18 into the rooms 12, 14.

A refrigerant system is diagrammatically illustrated as including a compressor unit 26 that is driven by an electric motor 28 to compress refrigerant that is directed through a conduit 30 and thence through a condenser 32 and another conduit 34 through a refrigerator expansion means representatively shown as being a capillary'tube 36 thence through an evaporator 33 back to the inlet of the compressor 26 through a conduit 40; The closed serially arranged refrigerating system preferably has the evaporator 38 in direct convective heat transfer relationship with air being conditioned within the plenum 20 for passage exteriorly thereof for providing a desired cooling of air to be discharged into the regions being conditioned. In addition, the environmental control system includes a heat source 42 representatively illustrated as comprising a gas burner unit 44 supplied by an inlet line 46 under the control of a valve 48, for example, a solenoid actuated valve selectively opened and closed in response to temperatures sensed by a room thermostat 47 having circuit contacts 49. The combustion products of the gas heater 44 are passed in direct heat transfer contact with air in the plenum 20 and thence through a suitable waste chimney 50 by conduit (not shown) to effect a desired heat input into the plenum 20.

In accordance with certain of the principles of the present invention, an improved control system is associated with the representatively illustrated heating and cooling systems for regulating it to produce a continuously modulated source of conditioned air into the regions 12 and 14 in a manner to produce a year around comfort control having substantially no temperature lag or overshoot from a preselectable comfort control point.

The improved control system is generally designated in FIGURE 2 as including a source of power 51 for driving the refregierant compressor motor 28 and the fan motor 25 under the control of a static solid state computer and power control package 52 which is associated with a comfort control programmer 54 which conditions a feedback signal from speed sensor means 56 from the compressor motor 28 for regulating the power control package 52. A like speed sensor means 57 is associated with the fan motor 25 to send a signal to programmer 54 for modulating the control action thereof.

In the representative illustration of the improved system, the computer control unit 52 receives information from variable resistance

temperature sensing units

58, 60, 62 located in the conditioned regions 12, 14 and the exterior environment around the housing unit as well as the signal from the speed sensing means 56 under the control of the comfort control programmer 54. Other suitable controls such as humidity can also be fed into the computer unit 52, if desired. The signal receiving computer unit 52, as will be described in greater detail, determines the power to be applied to the compressor motor 28 and blower motor 25 in accordance with the input signals from the sources mentioned above.

One feature of the present invention is that the computer of the control system for modulating heating and cooling effects by varying the power to the

motors

25, 28 includes static state semiconductor means that can be arranged in very compact control packages easily mounted with respect to the environmental control system. Additionally, such static semiconductor means establishes an improved reliability in the system insofar as its life expectancy is concerned. Another feature of the invention is that

motors

25, 28 are of the AC induction type. Such motors have been found to be unusually suited for systoms of the above discussed type in that they are unusually economical as compared to like horsepower universal and DC motors. Furthermore, AC induction motors are preferred because of their good torque characteristics over a wide range of operating speeds.

The speed discriminator section or speed sensing means 56, as shown in detail in FIGURE 3, includes a permanent magnet pickup means similar to a type used on dynamometers and includes a number of turns of wire 64 wound on a small magnet 66 located radially in alignment adjacent to a toothed wheel means 68 suitably secured for rotation with the shaft of the compressor motor 28. The above-described speed signal sensing means is located preferably inside a sealed motor compressor shell. The speed signal produced by the magnetic pickup is rectified by a full-wave rectifier bridge '70 and filtered by a filter unit 72 and passed to a filter unit 74 into the computer control unit 52.

In accordance with certain of the principles of the present invention, the comfort control point for a given heating or cooling operation is controlled by means including a summer comfort control indicating member 76 and a winter comfort control member 78 that have suitable indicia thereon such as early summer, mid-summer, late summer in the case of the summer control in early spring, late fall and winter in the case of winter control. These

control members

76, 78 enable the heating and cooling effects of the refrigerant system and heat source, respectively, to be selectively infinitely varied within a range limited by the maximum speed of compressor and

blower motors

28, 25 and a minimum speed determined by design conditions such as motor heating. Such control of heat and cold input into the mass of air enables the system to operate substantially continuously whereby a desired predetermined control point can be maintained without any. substantial lag or overshoot, notwithstanding changes in ambient conditions interiorly of the environment being conditioned and changes in heat loads being introduced into the environment being conditioned.

Referring now to FIGURE 4, the control for the environmental conditioning system is more detailedly set forth with the variable speed compressor motor 28 and blower motor 25 being shown powered from a 115-volt residential power source including a line L connected to a conductor 80 that supplies power to the

AC induction motors

25, 28 which are representatively illustrated as being of a permanent capacitor start

type having conductors

82, 84 connecting the motors, respectively, to output terminals of the computer control unit 52 which has another output terminal thereof connected to another conductor 86 that connects to a line N of the power source. Power from lines L N through

conductors

80, 86 is selectively controlled by a line switch 87.

Referring now more particularly to the comfort control programmer 54, in FIGURE 4 a switching device is illustrated including a plurality of relay actuated

switches

88, 90, 92, 94 operatively associated with a crossover relay 95 that is selectively actuatable in response to a predetermined temperature range for selectively connecting the heating system and cooling system in circuit with the computer 52. The relay includes a temperature responsive element 96 representatively illustrated as being a high positive coefiiicent of resistance element such as a thermistor that reduces current flow across a coil 97 as the temperature increases. The relay 94 is representatively illustrated as being included in a conductor 98 from the conductor 86. The thermistor element 96 preferably is located to sense the environment temperature somewhere between the regions 12, 14 at a point reflecting somewhat average conditions therein. 7

In operation, the relay 95 is calibrated to switch the heating and cooling systems into circuit with the computer 52 at the limits of a predetermined temperature range. For example, if it is desired generally to control the regions being conditioned within a 10 F. range, when the temperature sensed by the resistance element 96 is at either end of this range, for example at F. or F., the relay will switch one or the other of the systems into circuit with the computer 52 in the following manner. Assuming that the heating system is in circuit with the computing circuit 52, the relay 95 is conditioned so that the relay switches 88 and 92 are closed and the relay switches 90 and 94 are opened. If the temperature of the region being conditioned reaches 80 as might occur when the outside temperatures are in the 90 range, the relay coil 97 will be de-energized and the

switches

88, 92 will open and the relay switches 90, 94 will close to include the cooling system in circuit with the computer 52. Similarly when the cooling system is running and the temperature of the conditioned region reeaches 70 F. as might occur when the outside temperature begins to drop, the relay 95 will condition

switches

88, 90, 92, 94 for heating operation.

More specifically, during winter seasonal operation, the comfort control programmer 54, because of the aforementioned action of the relay 95, is conditioned to complete a winter seasonal control circuit therein including one terminal of a magnetic pickup winding 98 in speed sensor means 57 like winding 66 in sensor means 56 through a conductor 99, through the closed relay switch 88 and thence to a resistance element 100, a movable contact carrying arm 102 driven by the manually adjustable winter control member 78 relative to a variable resistance 104 of a potentiometer 105 that has a greater or lesser portion of the resistance thereof thereby placed in series with conductor 106 and conductor 108 electrically connected to a terminal of the input bridge 70.

By virtue of the above-described winter seasonal control circuit, a user of the environmental control system can modify a voltage from winding 48 produced by rotation of a toothed wheel 107 relative to a magnetic pickup 109 of the speed sensor 57 to thereby control the voltage input to the blower motor 25 and hence increase the rate of circulation of heat or reduce it to correspond to a desired comfort control condition as set by the control member 78.

When the relay 95 conditions the comfort control programmer for summer operation, the switch 88 is opened and switch 90 is closed whereby a summer seasonal control circuit is completed including a conductor 110, the

closed relay switch 90, a conductor 111, a movable contact carrying arm 112 that is moved with respect to a variable resistance portion 114 of a summer control potentiometer 116 upon adjustment of the summer control member 76. From the variable resistance 114, the circuit is connected by a conductor 118 to the conductor 108 and thence into one of the input terminals of the bridge 70. By virtue of thi arrangement, the summer temperature in the environment being conditioned can be adjusted to a predetermined comfort control point by manipulation of the control member 76 which produces a predetermined change of the speed input signal from the coil 64, as in the case of the winter seasonal control circuit. A calibrating potentiometer 120 is connected across the

conductors

99, and 108 for adjusting the input signal to the computer control unit 52 when the control system is originally installed.

In the illustrated embodiment of the invention signal input from the magnetic speed pickup means 56 and 57 as modified by the comfort control programmer unit 54, is rectified and filtered by the bridge 70 and capacitive filter unit 74, and then directed through a solid state semiconductor network including a unijunction transistor 122, connected emitter 1222 to base 122b across a capacitor 124. The signal from the capacitive filter 74, in response to variations in the input signal from the magnetic speed pickup means 56, 57 as modified by the comfort control programmer 54, regulates firing of the transistor 122 in combination with a control potential produced by a bridge circuit 126 that rectifies the A.C. power source across

lines

82, 84, 86 to D.C. power with the voltage across the output of bridge 126 being regulated by

Zener diodes

128, 130 in conjunction with

resistances

132, 134. Depending upon the imbalance between the control potential produced by the bridge 126 that is fed through a filter '72 to the filter 74 and the variable input signal from the bridge 70 and capacitive filter 74, the unijunction transistor 122 fires to produce a motor control signal in a primary coil 136 of a pulse transformer 138. Firing of the transistor 122 allows the capacitor 124 to discharge through a low impedance path from the transistor emitter 122e, base 122b and the primary coil 136 to produce a voltage spike therein. The voltage spike acts as a control signal that serves as the intelligence for operating a solid state power supply switch for supplying power to the

motors

25, 28 with the power supply switch being representatively illustrated as including a pair of oppositely facing silicon controlled

rectifiers

140, 142 which are actuated by the signal induced in the

secondary coils

144, 146 of the pulse transformer 138 to be energized for a predetermined period of the full wave form of the AC. power supply across lines L N to produce a variable energization of the blower motor 25 and compressor motor 28 to hence control the operative speed thereof.

During both the operation of the heating and coo-ling systems, it is desirable and sometimes preferable, depending upon the type of environment being conditioned by the system, to feed additional input intelligence to the computer unit 52 to modify the output signal thereof for varying the speed or blower and compressor motors to compensate for variances in conditions sensed by the means supplying the additional intelligence. For example, in the illustrated embodiment of the invention, the

temperature sensors

58, 60, 62 are included in parallel with the resistance 132 in the circuit for controlling the firing of the transistor 122 and the consequent triggering of the silicon controlled power switches 140, 142 to slow down and speed up the compressor motor and the blower motor to compensate for anticipated changes in the ambient conditions outside the house as well as to take into account change in the heating load within the regions 12, 14 being conditioned by the system. For example, in the illustrated arrangement, the

sensors

58, 60, 62 are high positive coefiicient of resistance elements such as thermistors that will act to increase the motor speed upon increases in temperature to produce an additional cooling effect and greater circulation of the mass of air being cooled to compensate for the increased temperatures. When the relay switches the system to winter operation, these elements are taken out of the circuit by a relay switch 148 which acts to include in

circuit winter sensors

149, 150 located in regions 12, 14 in the control circuit. These sensor are negative coeflicient of resistance elements. While the summer and winter sensing units are illustrated as being positive and negative coefiicient of resistance elements, respectively, it will be appreciated that opposite temperature coefficient of resistance elements could likewise be included in the system by making appropriate changes in the representatively illustrated solid state computer unit.

If desired, humidity sensing units can also be incorporated in the system to modify the action of the solid state computer unit whereby, upon an increase in humidity, the compressor motor speed will be increased to fur- I ther cool the evaporator 38 of the refrigerant system which may serve as a moisture extracting device by freezing the moisture in the circulated mass of air as it passes thereover. In this case, as is Well known in the art, suitable heating means can be included adjacent the evaporator to periodically melt the ice on the evaporator for passage into a suitable drain.

By virtue of the above-described control system, a user merely has to set a desired summer and winter comfort control point on the

seasonal control members

76, 78. During summer, the summer control circuit of the programmer 54 will regulate the input signal of the magnetic pickup on the compressor motor to maintain the speed of the motor at a basic control point that is modified by the temperature sensors in the environment being treated whereby the compressor motor speed will vary about the motor speed norm of the set comfort control point to produce a desired cooling of the mass of air being circulated across the evaporator 38 to meet the cooling demand. The provision of the

separate sensing elements

58, 60, 62 will continually vary the speed of the compressor motor on either side of the set speed norm to anticipate increases in outside temperature and changes in the cooling load required in the regions 12, 14 to effectively maintain a desired environment in the regions. The changes in speed of the motor compressor are followed by like changes in speed of the blower motor so that the additional cooling effect is manifested in the regions being treated by a corresponding increase in the rate of circulation efiected by the fan 24.

When the system automatically crosses over to winter operation as, for example, when the temperature sensed by the crossover relay switch reaches 70 F., the winter seasonal control circuit is closed and the heat source representatively shown as a gas burner 44 is turned on by closing relay switch 92 to complete a gas valve energization circuit 150 to solenoid 48 which includes the thermostat contact 49. Heat regulation is obtained by suitable means as, for example, a temperature sensor in the bonnet of the furnace that can be included as a high positive coefiicient of resistance element such as a thermistor 152 that is included in the conductor 108 on the winter seasonal control circuit so that the speed of the blower will increase upon increases in the bonnet temperature whereby the elfective B.t.u. output of the furnace is distributed quickly to the regions being treated to prevent any substantial change from the control point established by the thermostat 47. As was pointed out above, during the winter operation the compressor motor 28 is disconnected from the power supply and the blower motor speed 25 is thereby modified by changes in the resistance of the bonnet temperature sensing element 152 as fed into the computer from the programmer 54. Also during this operation, the blower speed sensor means 57 produces a speed modified by programmer 54 to eifect sensor speed control during late fall, early spring and the like.

While the representative environmental conditioning system is illustrated as including a separate source of heat, it will be appreciated that the refrigerant system can be operated as a heat pump merely by the provision of suitable reversing valves therein which can be conditioned by an automatic crossover switch or suitable manual selector switch means to provide the desired heat input into the system.

It will also be recognized that the comfort programmer and solid state computer unit are adaptable for use with a separate refrigerant system for conditioning a predetermined environment in which case the heater system and circuitry therefor would be eliminated or vice versa. One advantage of the integrated heating and cooling systems, however, is that a single solid state semiconductor computer unit serves to control the operation of both the heating and cooling units in a reliable and economical fashion.

The basic concept of a domestic appliance having an AC induction motor controlled by a static solid state or semiconductor computer control unit is set forth in United States Patent No. 3,152,462, issued Oct. 13, 1964, to Elliott et al. The present invention incorporates the basic advantages of appliance control by such static control devices, namely greater control reliability and more exact control capability together with the further improvement comprising additional comfort programmer means for correlating the home heating and cooling operations to varying heating and cooling loads.

While the embodiment of the present invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. In a household conditioning system the combination of, a refrigerant system including a compressor, a condenser, an evaporator, and means for connecting said compressor, condenser and evaporator in refrigerant flow relationship, enclosure means for receiving air returned from the rooms of a household and for supplying air thereto, said enclosure means defining a plenum in heat transfer relationship with said evaporator, fan means for directing conditioned air exteriorly of said plenum and for returning air thereto, mean for actuating said compressor and said fan means including prime mover means, semiconductor switch means in power supply relationship with said prime mover means, means for deriving a control voltage responsive to the speed of said prime mover means, means processing said control voltage for controlling conductivity of said semiconductor switch means to maintain said prime mover means at a predetermined speed norm, and comfort condition programmer means for infinitely varying said control voltage within predetermined limits to operate said prime mover means at variable speeds for producing variable cooling effects.

2. In a household conditioning system the combination of, a refrigerant system including a compressor, an evaporator and a condenser, means for connecting said compressor, condenser and evaporator in refrigerant flow relationship, means including an air supply and return for directing air to and returning air from rooms in the household, said air supply and return being located in heat transfer relationship with said evaporator, fan means for circulating conditioned air through said air supply and return, means including electric motor means for driving said compressor and said fan means, a control system means for variably modulating the cooling effects of said evaporator including a control circuit having semiconductor switch means therein for selectively conducting current from a power source to said electric motor means,

means for deriving a control voltage responsive to the operation of said compressor, means for processing said control voltage to regulate the conductivity of said semiconductor switch means to maintain said electric motor means at a predetermined speed norm.

3. In the combination of claim 2, comfort condition programmer means for infinitely varying said control voltage within predetermined limits for varying the speed norm of operation of said motor means in accordance with predetermined desired cooling conditions.

4. In the combination of claim 2, said means for processing said control voltage including means adapted to sense atmospheric conditions interiorly and exteriorly of the rooms being conditioned for varying conductivity of said semiconductor switch means and energization of said electric motor means thereby to vary the speed of operation thereof from said predetermined speed norm.

5. In an environmental conditioning system the combination of means including first electric motor means for cooling a mass of air, means including second electric motor means for circulating the mass of air for conditioning a predetermined environment, circuit means for selectively connecting said first and second motor means in power supply relationship with a source of power, computer control means operatively connected to said circuit means, said computer control means including semiconductor switch means for supplying power to at least one of said first and second electric motor means, means for deriving a control voltage responsive to the speed of said at least one of said electric motor means, said computer control means including means for processing said control voltage for controlling conductivity of said semiconductor switch means to maintain at least one of said motor means at a predetermined speed norm, and comfort control programmer means for infinitely varying said control voltage within predetermined limits to operate said at least one of said motor means at variable speeds for producing a modulated cooling of the mass of air to maintain predetermined environmental conditions preset by said programmer means.

6. In an environmental conditioning ystem the combination of, means for cooling a mass of air, means including electric motor means for circulating the mass of air in heat transfer contact with said cooling means and distributing the mass of air to an environment to be conditioned, circuit means for selectively operating said cooling means, said circuit means including computer control means having semiconductor switch means in power supply relationship with said motor means, means for deriving a control voltage in response to predetermined operating conditions of said motor means, said computer control means including means for processing said control voltage to control the conductivity of said semiconductor switch means for maintaining said electric motor means at a predetermined speed norm, and comfort control means for varying the conditioning effect of said cooling means by infinitely varying the effect of said control voltage on said semiconductor switch means within predetermined limits.

References Cited UNITED STATES PATENTS 3,010,289 11/1961 Kuklinski 62--215 X 3,182,222 5/1965 Lacy et al 317148.5 3,204,423 9/1965 Resh 62-215 3,214,931 11/1965 Petranek 62-215 X ROBERT A. OLEARY, Primary Examiner.

M. A. ANTONAKAS, Assistant Examiner.

Claims

2. IN A HOUSEHOLD CONDITIONING SYSTEM THE COMBINATION OF, A REFRIGERANT SYSTEM INCLUDING A COMPRESSOR, AN EVAPORATOR AND A CONDENSER, MEANS FOR CONNECTING SAID COMPRESSOR, CONDENSER AND EVAPORATOR IN REFRIGERANT FLOW RELATIONSHIP, MEANS INCLUDING AN AIR SUPPLY AND RETURN FOR DIRECTING AIR TO AND RETURNING AIR FROM ROOMS IN THE HOUSEHOLD, SAID AIR SUPPLY AND RETURN BEING LOCATED IN HEAT TRANSFER RELATIONSHIP WITH SAID EVAPORATOR, FAN MEANS FOR CIRCULATING CONDITIONED AIR THROUGH SAID AIR SUPPLY AND RETURN, MEANS INCLUDING ELECTRIC MOTOR MEANS FOR DRIVING SAID COMPRESSOR AND SAID FAN MEANS, A CONTROL SYSTEM MEANS FOR VARIABLY MODULATING THE COOLING EFFECTS OF SAID EVAPORATOR INCLUDING A CONTROL CIRCUIT HAVING SEMICONDUCTOR SWITCH MEANS THEREIN FOR SELECTIVELY CONDUCTING CURRENT FROM A POWER SOURCE TO SAID ELECTRIC MOTOR MEANS, MEANS FOR DERIVING A CONTROL VOLTAGE RESPONSIVE TO THE OPERATION OF SAID COMPRESSOR, MEANS FOR PROCESSING SAID CONTROL VOLTAGE TO REGULATE THE CONDUCTIVITY OF SAID SEMICONDUCTOR SWITCH MEANS TO MAINTAIN SAID ELECTRIC MOTOR MEANS AT A PREDETERMINED SPEED NORM.